Although solar radiation is a high-temperature and high-power heat source at its origin, its use in the flux conditions when it arrives at the Earth's surface almost eliminates its potential to be converted to work, due to the dramatic reduction in the temperature available in the flux. For this reason, thermoelectric solar plants (TSP) resort to optical concentration systems that allow obtaining greater flux densities and thereby higher temperatures. Consider a simplified model of a thermosolar concentration system consisting of an ideal optical concentrator, a solar receiver that behaves as a black body with losses due only to emission (a cavity receiver or a volumetric receiver will theoretically approach this condition) and a thermal machine or motor with an ideal Carnot efficiency. The total yield of this system will mainly depend on the efficiency of the receiver system and of the thermodynamic cycle. The efficiency of the solar receiver system can be expressed in a very simplified manner by Equation 1, where Qin is the inbound energy supply and Qloss is the thermal losses of the system. As the inbound energy is proportional to the concentration and thermal losses depend mainly on the process temperature, it can be said that the higher the concentration, the greater the efficiency of the solar receiver system for a given process temperature. In other words, in order to work at high temperatures, to increase the efficiency of the receiver system we must increase the concentration. That is, the efficiency of solar receivers will increase with high concentrations and low process temperatures.
On another hand, Equation 2 shows the efficiency of an ideal thermodynamic cycle (the Carnot cycle), the efficiency of which increases with temperature. The overall efficiency of the thermosolar system will be determined by the product of the two, as shown in Equation 3. To increase the overall efficiency of the system we must increase the concentration, to allow working at high temperatures and high overall efficiency.
                              η          receiver                =                                            Q                              i                ⁢                                                                  ⁢                n                                      -                                          Q                loss                            ⁡                              (                T                )                                                          Q                          i              ⁢                                                          ⁢              n                                                          [        1        ]                                          η          Carnotcycle                =                                            T              -                              T                0                                      T                    =                      1            -                                          T                0                            T                                                          [        2        ]                                          η          TOTAL                =                                            η              receiver                        *                          η              Carnotcycle                                =                                    (                                                                    Q                                          i                      ⁢                                                                                          ⁢                      n                                                        -                                                            Q                      loss                                        ⁡                                          (                      T                      )                                                                                        Q                                      i                    ⁢                                                                                  ⁢                    n                                                              )                        *                          (                              1                -                                                      T                    0                                    T                                            )                                                          [        3        ]            
From a thermodynamic point of view, there are concentration limits that differ depending on whether the concentration is effected in three dimensions (3D) or in two dimensions (2D). Namely, the concentration limit depends on the acceptance angle, this is, the size, shape and distance of the object to be found, in this case the sun, whose angle is 4.65 mrad.
                                                        C                              max                ⁡                                  (                                      2                    ⁢                    D                                    )                                                      =                          n                              sin                ⁡                                  (                                      θ                    a                                    )                                                              ;                                                    C                                  max                  ⁡                                      (                                          2                      ⁢                      D                                        )                                                              ≤                              215                ⁢                                                                  ⁢                                  C                                      max                    ⁡                                          (                                              3                        ⁢                        D                                            )                                                                                            =                                          n                2                                                              (                                      sin                    ⁡                                          (                                              θ                        a                                            )                                                        )                                2                                              ;                ⁢                                  ⁢                              C                          max              ⁡                              (                                  3                  ⁢                  D                                )                                              ≤          46000                                    [        4        ]            
As the theoretical concentration limits are much higher in the case of three-dimensional concentration systems than in two-dimensional systems, why have early commercial experiences applying solar concentration systems to generate electricity been based on two-dimensional concepts, as with parabolic trough concentrators? The reason is that in two-dimensional concentrators it is possible to use single-axis tracking systems to reach concentrations on the order of 20-80× and working temperatures of about 400° C. Three-dimensional systems, however, require two-axis tracing systems, which are much more complex, and generally reach concentrations of 300-2000× and working temperatures of up to 1000° C. It should be recalled that there are currently three different techniques developed for use in Thermoelectric Solar Plants: parabolic trough systems, central receiver systems and Stirling dish systems. These use only the direct component of solar radiation, which requires them to have solar tracking devices.
1. In parabolic trough collectors (2D), direct solar radiation is reflected by parabolic mirrors that concentrate the radiation in a receiver or absorber trough in which flows a fluid that is heated by the concentrated solar radiation to maximum temperatures of 400° C. In this way, solar radiation is converted into thermal energy that is later used to generate electricity by a Rankine water/steam cycle. A variation of this technology is Fresnel linear concentration systems, in which the parabolic mirror is replaced by a Fresnel discrete array with smaller mirrors that can be flat or have a slight axial curvature; controlling their axial orientation allows concentrating solar radiation on the absorber tube, which in this type of applications is generally stationary.
2. Central receiver systems (3D) use large mirrors (40-125 m2 each) known as heliostats, provided with a control system for reflecting direct solar radiation on a central receiver placed at the top of a tower. In this technique, the concentrated solar radiation heats a fluid in the receiver to temperatures of up to 1000° C. and this thermal energy is then used to generate electricity.
3. Stirling dish systems (3D) use a surface of mirrors mounted on a parabola of revolution to reflect and concentrate sunlight at a focal point where the receiver is placed, heating the working fluid of a Stirling engine that is used to drive a small electric generator.
Although all of the aforementioned techniques are at an early commercialization stage and it is too early to give conclusive cost estimates, we may say a priori that three-dimensional concentrator systems allow reaching higher working temperatures, and thus will increase the efficiency of the thermodynamic cycle employed, although in order to reach these concentrations they require high-precision 2-axis tracking systems, which can increase the cost per m2 built with respect to two-dimensional concentrator systems.
Therefore, an object of the present invention is to provide a system that is technically more efficient and economically more competitive.